DESCRIPTION (Applicant's Description) Short term increase in breast cancer following pregnancy suggests that the events of pregnancy promote growth of occult tumors. The mechanism for tumor promotion in pregnancy is unknown. However, maternal immune changes that allow tolerance of the pregnancy could decrease immune competence against a developing breast cancer. Precedence exists for global changes in immune function during pregnancy as evidenced by remission of rheumatoid arthritis in pregnancy, a cell mediated autoimmune disorder. Unfortunately, there is no simple marker for immune activation. However, characteristics of the placenta, which is the source of possible immune challenge to the mother, may be epidemiologic markers for immune activation. The proposed study would be the first epidemiologic investigation of the relation of placental characteristics to breast cancer risk. This study is expected to 1) identity new markers of high risk; 2) advance understanding of the etiology of short- term risk of breast cancer following pregnancy and 3) provide preliminary analyses for a larger research project which will explore the role of cytokines as markers for breast cancer risk following pregnancy. The study has the following aims: 1) Investigate the hypothesis that placental characteristics and associated clinical conditions of pregnancy are prospectively associated with breast cancer. 2) Test the feasibility of measuring cytokines in stored CHDS sera. Significantly, this important investigation can be accomplished at relatively low cost because it makes efficient use of a unique existing study population and data set: The Child Health and Development Studies (CHDS). These data consist of a 30 year follow-up of a cohort of approximately 15,000 women who were pregnant in the 1960's. Nearly 400 cases of breast cancer have been identified in this cohort. Frozen sera are available. The results of the proposed study will be used to guide the design of a larger study of the relation of cetaceans immune response markers to breast cancer risk. The proposed preliminary study will assure that the valuable CHDS serum samples will be used to investigate a hypothesis that shows promise for advancing knowledge of breast cancer etiology and identifying new markers of risk.